they used to provide strategic advice to assad regime. nowadays, they are even leading operations. >> narrator: opposing iran is saudi arabia, which is backing the sunni rebels. >> the rivalry between saudi arabia and tehran had started long before the syrian uprising. and now with the syrian uprising, the saudi government saw this as an opportunity to deal a mortal blow to iran. and they have carved that niche in this conflict by sending weapons and money to the rebels. (chanting) (explosion) but as long as they have the iranian support, i think this regime can continue to fight for some time. >> the potential for it to turn into a very dark and tough sectarian fight, the way it did in lebanon and iraq, is very high. and the regime has made it very clear that, "we're not going anywhere. and we're going to fight to the end. and if you want to take us on, you have to be prepared to sacrifice everything, and you may not win." it's hard to see where this ends. (gunfire) >> frontline continues online with more frguardian reporter ghaith abdul